1. Field of the Invention
This invention relates generally to a torque converter for an automatic transmission, and, in particular, to determining the rotating speed of a torque converter impeller without a speed sensor for this purpose.
2. Description of the Prior Art
A torque converter is a modified form of a hydrodynamic fluid coupling, and like a fluid coupling, is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load. A torque converter is able to multiply torque when there is a substantial difference between input and output rotational speed, thus providing the equivalent of a reduction gear.
In a torque converter there are at least three rotating elements: the impeller, which is mechanically driven by the prime mover; the turbine, which drives the load; and the stator, which is interposed between the impeller and turbine so that it can alter oil flow returning from the turbine to the impeller. The classic torque converter design dictates that the stator be prevented from rotating under any condition, hence the term stator. In practice, however, the stator is mounted on an overrunning clutch, which prevents the stator from counter-rotating the prime mover but allows for forward rotation.
In a torque converter there are at least three rotating elements: the impeller, which is mechanically driven by the prime mover; the turbine, which drives the load; and the stator, which is interposed between the impeller and turbine so that it can alter oil flow returning from the turbine to the impeller. The classic torque converter design dictates that the stator be prevented from rotating under any condition, hence the term stator. In practice, however, the stator is mounted on an overrunning clutch, which prevents the stator from counter-rotating the prime mover but allows for forward rotation.
Losses within the torque converter reduce efficiency and generate waste heat. In modern automotive applications, this problem is commonly avoided by use of a lock-up clutch, which that physically links the impeller and turbine, effectively changing the converter into a purely mechanical coupling. The result is no slippage, and therefore virtually no power loss and improved fuel economy.
The next generation of torque converter technology enables the decoupling of the impeller from the engine during idle, better known as idle disconnect. The decoupling of the impeller reduces engine load and thereby fuel consumption during drive and reverse idle. This decoupling is accomplished with an impeller clutch between the impeller and engine. The impeller clutch is controlled by a pressure differential between converter charge and discharge circuits.
To control accurately engagement, disengagement and slip of the impeller clutch, it is necessary to know the rotating speed of the impeller.
When the impeller clutch is slipping, impeller speed is less than engine speed. In order to control the slip across the impeller clutch real time impeller speed is required. Impeller speed can be determined from a signal representing impeller speed produced by sensor located in the torque converter. This requires a high powered sensor in the bell housing and forming the converter housing of a nonmagnetic material such as aluminum or stamping the housing from a sheet of low-carbon stainless steel. Low-carbon stainless steel, if heat treated properly, does not have magnetic properties and is transparent to a conventional sensor. This method requires a large air gap between the sensor and magnetic pick up on the impeller, causes the converter housing to be brittle, and adds complexity and cost to the components and assembly.
There is a need in the industry for a more cost efficient technique to determine the impeller speed continuously in real time.